This invention relates to a circuit device for cancelling out glitch pulses in a switched capacitor low-pass filter.
The invention also relates to a switched capacitor low-pass filter of a type which incorporates a cascade of integrator stages connected together and to the filter output, which output is fed back to the input of each stage.
As is well known in this specific technical field, analog switched capacitor filters are at present extensively used for many audio applications.
These filters are more compact and accurate than the equally well known continuous time filters.
FIG. 1 herein illustrates schematically an embodiment of a switched capacitor low-pass filter of the fourth order. This filter is used in a D/A (Digital-to-Analog) converter effective to re-construct an audio signal, e.g., for sigma/delta applications.
The filter shown in FIG. 1 is of the IFLF type and comprises a cascade of integrator stages connected together. In addition, the filter output is feedback connected to the input of each stage.
With the filter being of the fourth order, there will be four integrator stages connected in series.
More particularly, each integrator includes an operational amplifier having two inputs and two outputs, with each output being feedback connected to a corresponding input through an integration capacitor.
Additional sampling capacitors are provided between each input and a corresponding voltage reference. Also, a pair of switching capacitors are placed between the filter output and each integrator stage.
Within the integrator, plural microswitches are arranged to switch the connections between the various capacitors and the operational amplifier.
During a first step, the switching capacitors placed at the input of each integrator stage are sampling the voltage supplied from the previous stage. During a subsequent step, these switching capacitors will transfer their stored charge to the integration capacitor which is feedback connected between outputs and inputs of the operational amplifier.
The output voltage from each operational amplifier is held xe2x80x9cfrozenxe2x80x9d throughout the duration of the first or storage step, and will only change during the second or charge transfer step.
The succession of sampling and charge transfer steps result in a filtered signal being output which obeys the curve plotted in FIG. 2 for an ideal case with the input signal being sinusoidal.
When a case with real components is considered, however, a perturbed signal is always present at the filter output due to a series of glitch pulses, known as xe2x80x9cglitchesxe2x80x9d in the art, appearing as the microswitches close (Step 2 in FIG. 1). These pulses are repeated at each switching between the first and the second step as shown in FIGS. 3a and 3b, for example.
This effect is caused by the operational amplifiers of the integrator stages having a finite frequency response.
Normally, no serious side effects occur while the discrete time mode of operation is maintained. In fact, the switching capacitors would lock the value of the input voltage at the end of the first sampling step, that is, once the pulsive disturbance can be regarded to have been exhausted.
Where the discrete time filter is, as is often the case in the audio field, interfaced to a continuous time circuitxe2x80x94which usually is another low-pass filter intended for attenuating high-frequency harmonicsxe2x80x94then the effects of glitches may become serious.
In fact, the continuous time low-pass filter would then integrate the whole signal, and the glitch pulses along with it, thereby causing distortions to occur in the output signal.
To obviate this drawback, the prior art proposes a solution as schematically illustrated in FIG. 4.
A deglitching device or circuit is placed between the output of the switched capacitor filter and the input of the time continuous filter. This approach is a currently adopted one for commercially available D/A converters of the single-end type, although it can be used for differential applications as well.
While being in many ways advantageous and substantially achieving its objective, this prior solution cannot meet high performance requirements due to the noise introduced in the output signal.
The configuration shown in FIG. 4 actually exhibits overall noise which is the sum of the cascaded noise from the switched capacitor filter, the deglitching device, and the time continuous filter.
In essence, therefore, the signal-to-noise ratio provided by this solution is less than fully satisfactory.
An embodiment of this invention provides a circuit device which is effective to cancel out glitch pulses due to non-ideality of the operational amplifiers, at the output of a switched capacitor low-pass filter. This device has such constructional and functional features as to accommodate high signal-to-noise ratios resulting from high-performance filter operation and provides for a low introduction of noise, thereby removing the drawbacks that are besetting prior art approaches.
The device includes a smoothing integrator stage of the switched capacitor filter together with a deglitching device.